Thermo-sensitive recording media, such as thermal-sensitive recording paper, are disclosed, e.g., in JP-B-45-14035, which comprise a thermo-sensitive recording layer containing a colorless or lightly colored leuco dye and a color developer for developing color by reaction with the leuco dye upon heating. Such thermo-sensitive recording media are practically used widely. (The term "JP-B" as used herein means an examined Japanese patent publication.)
Thermal printers having thermal heads or the like apparatus are used for recording images on the thermo-sensitive recording media. The thermal recording method using such media is advantageous, in comparison with other conventionally practiced recording methods, in the points of loss noise generation during recording, no needs of development and fixation of images, freedom from maintenance of the apparatus, relatively low cost and compactness of the apparatus, and high sharpness of developed color. Therefore, the thermal recording methods are widely used for recording paper for output recording of computers, facsimiles, electronic calculators, measuring instruments, automatic ticket vending machines and the like.
In recent years, one application field of the thermo-sensitive recording media widely spreading is for labels as the result of increase of POS systems (point-of-sale systems). The thermo-sensitive recording paper for POS systems are mostly used for price indication and bar code indication for perishable foods, and may be brought into contact with water, foodwrapping films containing plasticizers, etc., oils, and the like. Therefore, the thermo-sensitive recording paper for such use is usually provided with a protective layer on the thermo-sensitive color-developing layer in order to prevent the penetration of the above-mentioned foreign matters (e.g., water, plasticizers and oils) into the color-developing layer, and/or with a back layer and/or an underlayer to pravent penetration of the foreign matters from the back face, so as to stabilize the formed images, as shown. e.g., in JP-A-57-188392. (The term "JP-A" as used herein means an unexamined published Japanese patent application.)
The thermo-sensitive recording label paper, as shown in JP-A-U-53-89334 for example, is provided on the back face with a pressure-sensitive adhesive layer which is covered by release paper having been subjected to release treatment. After printing is made, if necessary, on the front surface thereof, it is out into a label form with a die cutting machine having rotary cutters, etc. (The term "JP-A-U" as used herein means an unexamined published Japanese utility model application.)
Recently, in the die-cutting process, attempts are made to increase the cutting speed of label paper and to decrease the width of cut residue called a "skeleton" resulting from the cutting for the purpose of improving the yield and productivity. However, such rationalization of the cutting process leads to increase of the mechanical impact force given to the skeleton on cutting, which is liable to cause breakage of the skeleton, and to make difficult the removal of the skelton from the label, thus disadvantageously resulting in low productivity.
As the recording property among the characteristics required for the labels, high concentration of developed color with low energy consumption, namely high sensitivity, has come to be desired because of the trend of compacting and energy-saving of the recording apparatuses such as bar code printers. The further sensitization is of supreme difficulty because, as being evident from the layer structure of the aforementioned thermo-sensitive recording label paper, the protective layer on the thermo-sensitive color developing layer impairs the efficiency of heat conduction.
For improvement of the sensitivity, various proposals have been submitted regarding the leuco dyes, and color developers used in the thermo-sensitive color developing layer as well as sensitizing agents. Since the decline of the heat conduction efficiency cannot be avoided, it is more important to utilize most effectively the thermal energy given by a thermal head, or in other words, to improve the contact between the thermal head and the surface of the thermo-sensitive recording label paper.
Regarding the improvement of the above-mentioned contact between the thermal head and the recording paper, JP-B-52-20142, for example, describes a method of treating the surface of thermo-sensitive recording paper with a super-calender to attain a Bekk smoothness of the recording paper of from 200 to 1,000 seconds. JP-A-61-179786 describes the use of a support containing a pigment in an amount of 10 wt % or more and having an internal bond strength of from 0.5 to 2.5 kg-cm measured according to Tappi RC-308 to improve the contact with the thermal head. That is, it proposes a method for increasing the flexibility by using a support having a weak internal bonding force and by making closer the contact with the thermal head to improve the recording sensitivity. The above proposals, however, concerns with a thermo-sensitive recording medium being constituted only of a support and a thermo-sensitive color-developing layer provided thereon, but does not concern with thermo-sensitive recording mediums having a protective layer provided on the color-developing layer.
As mentioned above, in recent years, the thermo-sensitive recording label paper is urgently demanded to satisfy simultaneously two requirements of higher processability and higher recording property. Nevertheless, no investigation has been made regarding the mechanical characteristics in die-cutting nor improvement of recording sensitivity in a thermo-sensitive recording label form. Accordingly, no thermo-sensitive recording label paper has been developed which simultaneously satisfies the requirements of the processing characteristics and the recording characteristics.